This disclosure relates to a liquid level sensor for detecting a remaining amount of a liquid fuel in a fuel tank mounted on a vehicle. In particular, it relates to a sliding liquid level sensor having a contact point sliding over a plurality of electrodes in accordance with the movement of a float.
In conventional sliding liquid level sensors, silver often serves as major component in alloys, such as silver-palladium alloy or silver-nickel alloy. They are typically used for a sliding contact point and electrodes. Silver has low electrical resistance and thus has good conductivity. Silver, however, can get corroded through contact with a liquid fuel component such as those containing sulfur, water or alcohol. Such deterioration or corrosion can cause the connection to become loose. In particular, silver easily reacts with sulfur, thereby forming a silver sulfide membrane on surfaces of the sliding contact point and the electrodes. Since silver sulfide is an insulator, the silver sulfide membrane causes a loose connection, resulting in an eventual decrease in detection accuracy.
Disclosure of Japanese Laid-Open Patent Publication No. 2007-187633 tends to solve such a problem. In Japanese Laid-Open Patent Publication No. 2007-187633, electrodes are made from a silver-palladium alloy with glass and the contact point is made from a copper-nickel alloy. The electrodes contain 30-50 parts by weight of glass per 100 parts by weight of the silver-palladium alloy, and their silver/palladium ratio by weight is between 70/30 and 60/40. Copper/nickel ratio by weight of the copper-nickel alloy is 70/30, and the copper-nickel alloy has Vickers hardness between 190 and 250.
In Japanese Laid-Open Patent Publication No. 2007-187633, a large amount of glass is used for the electrodes, and silver is not used for the contact point in order to prevent loosening of the connection via formation of silver sulfide, etc. However, since the electrodes contain silver as a major component, it is not able to effectively prevent silver sulfide formation. In addition, the large amount of glass makes the electrodes too hard, so that it is necessary to harden the contact point in accordance with the hardness of the electrodes. Since silver cannot be used for the electrode, the contact point is made from the copper-nickel alloy having such the required level of hardness. However, copper, like silver is easily sulfurized or oxidized when coming in contact with a liquid fuel. Therefore, there has been need for improved sliding liquid level sensors.